Containerization-and-DevOps

Docker Compose

1. Theory: Docker Compose vs Docker Run

Docker Compose is a tool for defining and managing multi-container Docker applications. It uses a YAML configuration file (docker-compose.yml) to describe all the services, networks, and volumes your application needs, and then creates and starts them all with a single command.

What is Docker Run?

docker run is an imperative command used to launch a single Docker container. Every configuration option (ports, volumes, environment variables, network, etc.) must be passed as flags on the command line each time you run the container. This approach is verbose and error-prone for complex setups.

Example: Running Nginx with docker run
docker run -d \
  --name my-nginx \
  -p 8080:80 \
  -v ./html:/usr/share/nginx/html \
  -e NGINX_HOST=localhost \
  --restart unless-stopped \
  nginx:alpine

What is Docker Compose?

Docker Compose is a declarative approach — you define WHAT you want, not HOW to do it. All the same configuration that you would pass to docker run is written once in a YAML file and can be reused, version-controlled, and shared with team members.

# docker-compose.yml
version: 3.8

services:
  nginx:
    image: nginx:alpine
    container_name: my-nginx
    ports:
      - "8080:80"
    volumes:
      - ./html:/usr/share/nginx/html
    environment:
      - NGINX_HOST=localhost
    restart: unless-stopped

Key Advantages of Docker Compose

• Simplicity: One command (docker compose up -d) starts the entire multi-container application.

• Reproducibility: Same configuration runs identically on every machine — no forgotten flags.

• Declarative: Define what you want, not how to run it. Self-documenting configuration.

• Lifecycle Management: Easy commands to start, stop, view logs, and check status.

• Dependency Management: Use depends_on to control startup order between services.

• Scalability: Scale individual services with --scale flag.

Flag Mapping: docker run → docker-compose.yml

This guide provides a quick mapping between docker run command-line flags and their equivalent syntax in a docker-compose.yml file.

docker run Flag	Docker Compose Equivalent	Description
-p 8080:80	ports: ["8080:80"]	Map host port to container port.
-v ./data:/app	volumes: ["./data:/app"]	Bind mount a host directory or volume.
-e KEY=value	environment: ["KEY=value"]	Set environment variables.
--name myapp	container_name: myapp	Assign a specific name to the container.
--network net	networks: ["net"]	Connect the service to a specific network.
--restart always	restart: always	Set the restart policy.
-d	docker compose up -d	CLI Flag: Run in detached mode.
--link db	links: ["db"]	(Legacy) Explicitly link to another container.
N/A	depends_on: ["db"]	Startup order (wait for db to start first).
-w /app	working_dir: /app	Set the working directory inside the container.
--user 1000	user: "1000"	Run as a specific user (UID).

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2. Practical: WordPress + MySQL with Docker Compose

The docker-compose.yml File

The following compose file defines two services (wordpress and mysql), a shared network, and persistent volumes for both containers:

# docker-compose.yml
version: '3.8'

services:
  mysql:
    image: mysql:5.7
    container_name: mysql
    environment:
      MYSQL_ROOT_PASSWORD: secret
      MYSQL_DATABASE: wordpress
      MYSQL_USER: wpuser
      MYSQL_PASSWORD: wppass
    volumes:
      - mysql_data:/var/lib/mysql
    networks:
      - wordpress-network

  wordpress:
    image: wordpress:latest
    container_name: wordpress
    ports:
      - "8080:80"
    environment:
      WORDPRESS_DB_HOST: mysql
      WORDPRESS_DB_USER: wpuser
      WORDPRESS_DB_PASSWORD: wppass
      WORDPRESS_DB_NAME: wordpress
    volumes:
      - wp_content:/var/www/html/wp-content
    depends_on:
      - mysql
    networks:
      - wordpress-network

volumes:
  mysql_data:
  wp_content:

networks:
  wordpress-network:

Create the Compose File

nano docker-compose.yml

Command: nano is a terminal-based text editor. This command creates/opens the file docker-compose.yml for editing. Press Ctrl+X, then Y, then Enter to save and exit.

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Start the Containers

docker compose up -d

Command: docker compose up -d — Builds, (re)creates, starts, and attaches to containers defined in the compose file. The -d flag runs them in detached (background) mode, so your terminal is not blocked.

Expected output:

✔ Network devops_wordpress-network Created 0.1s
✔ Container mysql Created 0.2s
✔ Container wordpress Created 0.1s

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Port Conflict — Error and Fix

On the first attempt, an error occurred because port 8080 was already in use by another process on the host:

Error response from daemon: failed to set up container networking:

failed to bind host port 0.0.0.0:8080/tcp: address already in use

Explanation: Each port on a host machine can only be bound to one process at a time. Port 8080 was occupied by another container or service. The solution was to edit docker-compose.yml to change the host port (e.g., 8081:80) or stop the conflicting process.

To resolve, the containers were brought down, the compose file was edited to use a free port, then started again:

docker compose down

# (edit docker-compose.yml to change port mapping)

docker compose up -d

Command: docker compose down — Stops and removes containers, networks, and the default network created by 'up'. Volumes are preserved unless --volumes flag is passed.

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Check the webside

http://localhost:9090

If WordPress is working, that means:

• MySQL container is running
• WordPress container is running
• Network is working
• Volumes are mounted correctly

---

Attempt Service Scaling

docker compose up \--scale wordpress=3 -d

Command: docker compose up --scale <service>=<count> -d — Attempts to run the specified number of replicas of a service. This is useful for load testing or high availability setups.

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However, this produced a warning:

docker compose up --scale wordpress=3 -d
WARN[0000] /mnt/d/ccvt/sem 6/devops/docker-compose.yml: the attribute `version` is obsolete, it will be ignored, please remove it to avoid potential confusion
[+] up 1/1
 ✔ Container mysql Running                                              0.0s
WARNING: The "wordpress" service is using the custom container name "wordpress". Docker requires each container to have a unique name. Remove the custom name to scale the service

Explanation: The container_name: wordpress field in the compose file forces all replicas to use the same name. Since Docker container names must be unique, scaling fails. To enable scaling, remove the container_name field so Docker auto-generates unique names like devops_wordpress_1, devops_wordpress_2, etc.

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Tear Down

docker compose down

This cleanly removes all containers and networks created during the session. Final output:

✔ Container wordpress Removed 1.5s
✔ Container mysql Removed 1.8s
✔ Network devops_wordpress-network Removed 0.3s

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3. Quick Reference Cheatsheet: Docker Compose CLI

This cheatsheet covers the most frequently used commands for managing multi-container applications.

Command	Description
docker compose up -d	Start all services in the background (detached mode).
docker compose down	Stop and remove containers, networks, and images defined in the file.
docker compose logs -f	View real-time log output from all services (-f to follow).
docker compose ps	List the status of containers managed by the current project.
docker compose up -d --scale svc=3	Scale a specific service (svc) to 3 running replicas.
docker compose restart	Restart all containers in the project.
docker compose exec <svc> bash	Open an interactive bash shell inside a running service container.
docker compose pull	Download the latest versions of the images defined in your YAML.
docker compose build	Rebuild images defined with a build: context in the YAML.
docker compose stop	Stop services without removing the containers or networks.

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WSL Command

aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ nano docker-compose.yml
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker compose up -d
WARN[0000] /mnt/d/ccvt/sem 6/devops/docker-compose.yml: the attribute `version` is obsolete, it will be ignored, please remove it to avoid potential confusion
[+] up 3/3
 ✔ Network devops_wordpress-network Created                             0.1s
 ✔ Container mysql                  Created                             0.2s
 ✔ Container wordpress              Created                             0.1s
Error response from daemon: failed to set up container networking: driver failed programming external connectivity on endpoint wordpress (426628fa36705fa2a82e5b8ec5b3cc75293e1500dd2d2ccc1caa2d259ff2a041): failed to bind host port 0.0.0.0:8080/tcp: address already in use
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ nano docker-compose.yml
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker compose down
WARN[0000] /mnt/d/ccvt/sem 6/devops/docker-compose.yml: the attribute `version` is obsolete, it will be ignored, please remove it to avoid potential confusion
[+] down 3/3
 ✔ Container wordpress              Removed                             0.1s
 ✔ Container mysql                  Removed                             2.2s
 ✔ Network devops_wordpress-network Removed                             0.3s
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker compose up -d
WARN[0000] /mnt/d/ccvt/sem 6/devops/docker-compose.yml: the attribute `version` is obsolete, it will be ignored, please remove it to avoid potential confusion
[+] up 3/3
 ✔ Network devops_wordpress-network Created                             0.0s
 ✔ Container mysql                  Created                             0.1s
 ✔ Container wordpress              Created                             0.1s
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker compose up --scale wordpress=3 -d
WARN[0000] /mnt/d/ccvt/sem 6/devops/docker-compose.yml: the attribute `version` is obsolete, it will be ignored, please remove it to avoid potential confusion
[+] up 1/1
 ✔ Container mysql Running                                              0.0s
WARNING: The "wordpress" service is using the custom container name "wordpress". Docker requires each container to have a unique name. Remove the custom name to scale the service
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$ docker compose down
WARN[0000] /mnt/d/ccvt/sem 6/devops/docker-compose.yml: the attribute `version` is obsolete, it will be ignored, please remove it to avoid potential confusion
[+] down 3/3
 ✔ Container wordpress              Removed                             1.5s
 ✔ Container mysql                  Removed                             1.8s
 ✔ Network devops_wordpress-network Removed                             0.3s
aniket@Aniket:/mnt/d/ccvt/sem 6/devops$